Will Vs Be going to?

Examples:

(a) According to the weather report, it will be cloudy tomorrow.
(b) According to the weather report, it is going to be cloudy tomorrow.
(c) Be careful! You'll hurt yourself!
(d) Watch out! You're going to hurt yourself!

To express a prediction---either Will or Be going to is used:

When the speaker is making a prediction (a statement about something s/he thinks will be true or will occur in the future), either will or be going to is possible.

There is no difference in meaning between (a) and (b).
There is no difference in meaning between (c) and (d).

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(e)     (A) Why did you buy this paint?
         (B) I'm going to paint my bedroom tomorrow.

(f) I talked to Arslan yesterday. He is tired of taking the bus to work. He's going to buy a car. That's what he told me.

To express a prior plan--only Be going to is used.

When the speaker is expressing a prior plan ( something the speaker intends to do in the future because in the past s/he has made a plan or decision to do it), only be going to is used.
In (e): Speaker (B) has made a prior plan. She decided to paint her bedroom last week. She intends to paint her bedroom tomorrow.

In (f): The speaker knows Arslan's intention to buy a car. Arslan made the decision in the past and he intends to act on this decision in the future. Will is not appropriate in (e) and (f).
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(g)     (A) The phone is ringing.
         (B) I'll get it.

(h)     (A) I don't understand this problem
        (B) Ask your teacher about it. She'll help you.

To express Willingness---only Will is used.

In (g): Speaker (B) is saying: "I am willing, I am happy to get the phone." He is not making a prediction. He has made no prior plan to answer the phone. He is, instead, volunteering to answer the phone and uses will to show his willingness.

In (h): Speaker (B) feels sure aobut the tacher's willingness to help.
Be going to is not appropriate in (g) and (h)

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Cheers!
Naeem